The Maillard reaction is what happens when you cook something yummy like toast or bacon! It's a special reaction that happens between amino acids (the building blocks of proteins) and sugars when they get heated up together.
Think of it like two people giving each other a big hug - the amino acids and sugars come together and change into delicious brown and flavorful substances that make your food taste extra good.
That's why toast or bacon can start out looking kinda plain and pale, but then turn brown and smell amazing after they've been in the oven or on the stove for a while. The Maillard reaction is what's making all that happen!